1. Field of the Invention
The present invention relates to a direction finder or bearing detection system used in an Airborn Collision Avoidance System (hereinafter abridged as ACAS). The system is in general based on the principle of Air-Traffic-Control-Radar-Beacon-System (ATCRBS).
2. Description of the Prior Art
According to the operational principles of ACAS, presence of an approaching aircraft is at first coarsely identified by monitoring a response signal to an ATC transponder of the approaching aircraft. Then a MODE-A interrogation signal based on ICAO standard ANNX-10 is emitted automatically to request a response signal from the aircraft so as to confirm the existence of an approaching aircraft. If an approaching situation is identified and if the time before possible collision is judged to be less than 60 sec, emission of MODE-A and MODE-C interrogation signals is commenced. Information of the altitude of the approaching aircraft based on barometer reading and obtained from a response to the MODE-C interrogation signal is compared with the barometric altitude information of the one aircraft and the result is used for making a judgement whether to issue an instruction to steer toward more safer bearings. When there is no response to the MODE-C interrogation signal, such as for instance in the case of presence of incidental MODE-OFF request sent from a ground control station or in the case of gabbling, it is not possible to make a judgement for the steering instruction. Then only an Airborne Proximity Warning Indicator (APWI) alarm is delivered.
Such an existing system is not satisfactory in view of the importance of the security in the air traffic.